The present invention relates to an optical apparatus such as a video camera and a digital still camera, and more particularly, to an optical apparatus which performs focus control with a focus signal extracted from an image signal.
In a predominant auto-focus method for an optical apparatus such as a video camera, an object image is photoelectrically converted into an image signal by an image-pickup element, a focus signal representing the sharpness (contrast) of the image is extracted from the image signal, and the position of a focus lens is controlled so as to achieve the maximum level of the focus signal. This is referred to as a TV-AF (television auto-focus) method or a contrast detection method.
The focus signal also referred to as an AF evaluation value signal is provided by extracting a high-frequency component from the image signal through a band-pass filter. When an ordinary object image is picked up, the level of the signal is increased as the focus lens approaches an in-focus state, and the position where the level is at the maximum corresponds to the in-focus point. It is known that the level of the focus signal is abruptly changed when the central frequency of the band-pass filter is high in the band of the image signal and is gradually changed when the central frequency is low, as shown in FIG. 2.
The frequency characteristic of the band-pass filter for extracting the focus signal is set such that the level of the focus signal is adequately reduced by moving the focus lens within the depth of focus around the in-focus point. This enables clear determination of the peak of the focus signal, that is, the in-focus point. Even when the focus lens is located away from the in-focus point, the focus lens can be moved by an amount approximately equal to the depth of focus to provide obviously different levels of the focus signal, thereby allowing determination of an in-focus direction.
Recently, in addition to the conventional SD (standard-definition) image (720 H×480 V), the enhanced HD (high-definition) image (1440 H×1080 V) has been used, and video cameras therefor have been proposed. Specifically, in a proposed technique, an HD image signal is produced by an output from a first image-pickup area (1440 H×1080 V) of an image-pickup element and an SD image signal is produced by reducing an output from a second image-pickup area (1080 H×1080 V)
In video cameras, favorable auto-focus (AF) control needs to be performed regardless of whether the HD or SD image is used. Japanese Patent Laid-Open No. H07(1995)-107359 has disclosed a method in which an optical apparatus capable of converting an image signal formed by an image-pickup element into another image signal can produce an AF evaluation value signal from the image signal before the conversion to perform satisfactory AF control regardless of the HD or SD image.
With the method disclosed in Japanese Patent Laid-Open No. H07(1995)-107359, however, AF accuracy may be low in picking up an HD image. For example, when an image of an object at low contrast is picked up, it may be determined that an HD image is not in focus even when the AF accuracy is at a proper level for an SD image.
This is because the method disclosed in Japanese Patent Laid-Open No. H07(1995)-107359 cannot find a change in the focus signal level needed to perform AF control for the HD image when the object is at low contrast. In other words, even when the focus lens is driven within the depth of focus around the in-focus point, that in-focus point cannot be determined clearly.